REPRODUCTION 435 
In the seeds of the Gymnosperms the endosperm repro- 
sents the prothallium or gametophyte. 
The formation of the seed we have seen to depend 
upon the fusion of the sexual cells or gametes. This 
process is a very widespread one and is the starting point 
of the development of the young sporophyte in all plants 
{—@ de 
Fic. 180.—AnTuERozoips or Mogs (4) anD FErn (s). 
above the Thallophytes. The mode of bringing the gametes 
together varies with the habit of life of the plant. Where 
the male gamete is a motile antherozoid it makes its way to 
the oosphere by means of its cilia, which enable it to swim 
freely in water. In those forms with a terrestrial habit, 
such as the Bryophyta and Pteridophyta, in which the 
antherozoid is ciliated (fig. 180), fertilisation can only be 
Fic, 181.—DEvELOPMENT OF THE ANTHERIDIUM IN THE Fern. (After Kny.) 
brought about when the gametophytes are moistened, as is 
the case from time to time. The antherozoids sometimes 
arise in antheridia upon the same gametophyte as the arche- 
gonia with their oospheres, sometimes upon different ones. 
In the heterosporous forms of course the latter is always 
the case. A large number of such gametophytes, bearing 
male and female cells respectively, are always produced in 
